The DC-DC voltage converter of the present invention is of the inductive switched-mode type, i.e. with an induction coil as the reactive element. It can convert a continuous input voltage into a continuous output voltage of the converter that generally has a different value from the input voltage. The input voltage is supplied mainly by a battery or accumulator, which means that the value of the input voltage can decrease over time, particularly when the DC-DC converter is operating. However, the converter must be able to guarantee an output voltage at a determined value, normally independently of the drop in input voltage, at a minimum operating value, and as a function of the output load.
There exist three types of inductive converters. A first type of inductive converter is the Buck converter, which can supply an output voltage of lower value than the input voltage value. A second type of inductive converter is the Boost converter, which can supply an output voltage of higher value than the input voltage value. Finally, a third type of inductive converter is a combination of a Buck converter and a Boost converter. This third type of converter can either raise the output voltage level relative to the input voltage level, or lower the output voltage level relative to the input voltage level.
One important peculiarity of an inductive converter is that it can continuously regulate the conversion rate simply by adjusting the duty cycle of the clock signal. Depending upon the converter configuration, a first clock signal is used to control at least a first switch, in order to increase the current in a linear manner through the induction coil in a first phase T1. This current through the induction coil is drawn from a continuous input voltage source, which may preferably be a battery or an accumulator. A second clock signal is used to control at least a second switch, in order to reduce the current through the induction coil in a linear manner in a second phase T2, as explained below. Preferably, the current in the induction coil must decrease to zero value for a DC-DC converter operating in discontinuous mode.
One problem that generally arises is being able to control properly the duration of the second phase T2, so that the current in the induction coil is zero at the end of period T2. Usually, this period T2 is controlled by measuring the residual voltage across one terminal of the second switch responsible for decreasing the current in the induction coil, relative to earth. When this voltage is zero, this means that the current is also zero through the induction coil. However, the practical difficulty of such a measurement is that the residual voltage is very low, for example of the order of ten millivolts. This means using a comparator with a very low offset. Moreover, the comparator used has to be extremely rapid, otherwise the second phase T2 is liable to extend beyond its ideal value, and an inverse current may arise in the induction coil. This thus results in a loss in the converter's efficiency.
One technique for overcoming the aforementioned difficulty is disclosed in U.S. Pat. No. 7,279,877 relating to a Buck converter. In this patent, a comparator, which defines the instant at which the current in the induction coil is zero, has an offset adjusting device. Just after the end of period T2, the overvoltage across one of the poles of the induction coil is measured. The sign of this overvoltage is representative of the current that is flowing in the induction coil. Depending upon the overvoltage sign, the comparator offset is corrected, to adjust period T2 to a value corresponding to zero current in the induction coil. With this method, it is still necessary to use a rapid comparator for measuring the low residual voltage across the induction coil switch, as for the method of the aforecited prior art, which is a drawback.